This invention relates generally to gas analysis devices, specifically to the detection and monitoring of carbon dioxide gas through the use of non-dispersive infrared optical technology. The primary intended applications are the verification of proper intubation, assessment of the adequacy of ventilation (as in CPR), and the detection of cessation of respiratory activity (apnea).
Mechanical ventilating machines and hand bagging techniques both pump respiratory gases to and from a patient through an endotracheal tube that has been inserted into the patient's trachea. The ventilator output port is typically a plastic tube that slides onto the end of the endotracheal tube.
Prior art respiratory carbon dioxide analyzers generally fall into one of two categories. The first category is a sidestream system that extracts gas samples from the endotracheal tube and passes the sample through an optical cell within the instrument. The second category is a mainstream system with a precisely-dimensioned optical cell inserted directly in the flow path between the ventilator and the endotracheal tube. The output of both types of prior art analyzers is a precise indication of the shape and duration of the optical waveform and, in some, a precise measurement of carbon dioxide concentration in the respiratory gas.
U.S. Pat. No. 4,648,396, shows a system with an infrared source and an infrared detector mounted in two heads of a clothespin type clip-on device. The device clips onto a special Y-shaped connector which connects between the endotracheal tube, a source of inspired gas and a receptacle for expired gas. Recesses are provided in the Y connector for mating with the clothespin heads. This system, like other prior art systems, utilizes the fact that carbon dioxide absorbs infrared radiation at a wavelength of approximately 4.25 microns. However, this system provides only a qualitiative indication of relative changes in carbon dioxide concentration in the airway. Other related systems are shown in U.S. Pat. Nos. 4,417,589; 4,537,190; 4,938,703; 3,826,918; and 4,914,720.
One problem with measuring the gas content for a particular patient is that of contamination of the sensor. The invention disclosed in U.S. Pat. No. 4,648,396 avoids this problem by having the clothespin sensor clip on to the outside of a special Y-shaped connector with the Y-shaped connector having windows for the passage of infrared radiation. Thus, the sensor never comes in contact with the patient's breath. U.S. Pat. No. 3,826,918 shows the use of a porous window with purging fluids to avoid contamination. U.S. Pat. No. 4,914,720 shows in FIG. 8 a disposable plastic adapter with sapphire windows. The adapter is placed in the gas monitoring system of FIG. 2 with infrared radiation being projected through the sapphire windows.